Scientists at the University of Texas at Austin and colleagues studied the gut microbes in two species of fish and in mice, and also conducted an in-depth analysis of data that other researchers collected on humans.

They found that in fish and humans diet affected the microbiota of males and females differently.

In some cases, different species of microbes would dominate, while in others, the diversity of bacteria would be higher in one sex than the other.

"Our study asks not just how diet influences the microbiome, but it splits the hosts into males and females and asks, do males show the same diet effects as females?" said Daniel Bolnick, professor in the University of Texas at Austin's College of Natural Sciences and lead author of the study.

While the results identify that there is a significant difference in the gut microbiota for males and females, the dietary data used in the analysis are organised in complex clusters of disparate factors and do not easily translate into specific diet tips, such as eating more vegetables or less meat.

Why men and women would react differently to changes indiet is unclear, but there are a couple of possibilities, researchers said.

The hormones associated with each sex could potentially influence gut microbes, favouring one strain over another.

Also, the sexes often differ in how their immune systems function, which could affect which microbes live and die in the microbiome.

One notable exception in Bolnick's results was in the mice. Although there was a tiny difference between male and female mice, for the most part the microbiota of each sex reacted to diet in the same manner.

Since most dietary studies are conducted on mice, this result could have a huge effect on such research, and it raises questions about how well studies of gut microbes in lab mice can be generalised to other species, particularly humans.